Being old and local enough to remember Avebury when it was less popular than it is today, I have seen the way that popularity has affected the monument.
When it was less managed with no big tourist car park, I could cycle over to the the unkempt stones and sit or wonder among them in relative isolation with the odd visitor passing through. Today the tourists and day visitors arrive in their hundreds swarming half the circle (where the stones are) the majority of which are well behaved if not fully aware of what they are looking at. Most just see it as a photo opportunity or somewhere to take the kids to burn their energy. A few will be disrespectful and entice injury and possibly damage by climbing on the stone (and yes I still tell them to get off). However I rarely visit the circle during these times as I feel the place has lost the peace and solitude I enjoyed and I can no longer set up and take pictures without little Johnny tearing through the shot every two minutes. Instead I visit the less well known part of the complex or other less known places throughout the area. These places are just as interesting, look more natural and in the most part, are visitor free. Even when there are visitors they tend to be those with an interest and understanding of the monument and show the respect they deserve. I never mind these people even if they get in the way of my latest next photo. I often just wander around the place thinking or my next shot or taking in the place a little more than I probably would normally.
So my thoughts are let the tourists and day visitors go to the well managed premium sites, pay their overly inflated entrance fees or buy memorabilia from the visitors shop, but keep the Devils Den's, Lugbury's, and Lanhill's to those of us who know they are there and can be bothered to make the effort to visit and respect the sites and anybody else who visits.